President Barack Obama says U.S. airline security has been stepped up after explosives were found in packages shipped to the United States from Yemen. The president says the parcels were a credible terrorist threat.

President Obama told reporters Friday the packages were spotted in Dubai and at Britain's East Midlands Airport, north of London. He said they apparently contained explosives.

"Our intelligence and law enforcement professionals, working with our friends and allies, identified two suspicious packages bound for the United States, specifically two places of Jewish worship in Chicago," he said.

Watch Elizabeth Lee's Companion TV Report:

The president says U.S. intelligence and law enforcement reacted quickly. "I was alerted to this threat last night by my top counterterrorism adviser, John Brennan. I directed the Department of Homeland Security and all our law enforcement and intelligence agencies to take whatever steps are necessary to protect our citizens from this type of attack," he said.

Authorities say the packages had been shipped from Yemen, the base of an al-Qaida branch that tried to bomb a U.S.-bound airliner last December.

Mr. Obama says he has ordered an all-out investigation into the origins of the packages and their connection to any terrorist plot. "Although we are still pursuing all the facts, we do know that the packages originated in Yemen. We also know that al-Qaida in the Arabian Peninsula, a terrorist group based in Yemen, continues to plan attacks against our homeland, our citizens and our friends and allies," he said.

The president says his counterterrorism assistant, John Brennan, talked Friday with Yemen's President, Ali Abdullah Saleh, who Mr. Obama says has pledged his full cooperation in the investigation.

President Obama says U.S. counterterrorism officials are taking the threat seriously, and are taking all necessary and prudent steps to insure Americans' security.

Brennan told reporters the national security system worked well in response to the threat.